The overall goal is to study the mechanism and nature of molecular events involved in and leading to platelet aggregation. Specific areas to be investigated are: (1) Nature and control of platelet activation: a. Effect of aggregating agents and inhibitors on phosphorylation of intact platelets with (gamma-P32)ATP; b. Nature and properties of phosphorylated endogenous acceptors in intact platelets with (gamma- P32)ATP and Na3-P32O4 c. Nature and properties of phosphorylation of exogenous acceptors by intact platelets; d. Mechanism of phosphorylation (cAMP-dependent protein kinases and/or ATPases); e. Mechanism of dephosphorylation of endogenous acceptors. (2) Role of Ca in platelet activation and aggregation; (3) Nature of platelet cytogel extrusion; (4) Reversibility of platelet aggregation. Major techniques are conventional protein chemistry, methodology, electron microscopy, immunohistochemical techniques and typical isotope techniques.